Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface, showing the new design.

The broken line showing an electronic device is provided for the purpose of illustrating environmental structure. All other broken lines illustrate the display screen and elements of the graphical user interface. Any subject matter shown in broken lines forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 